按照Accelerated C++中的示例，我创建了一个自定义 STL 容器，它std::vector是Vec. 一切都很好，直到成功后，我尝试添加一个Vec::clear()可以清除矢量的东西。
这是最新的类定义（仅与此问题相关的部分）：
template <class T>
class Vec {
public:
Vec() { create(); }
size_type size() const { return avail - data; }
size_type capacity() const { return limit - data; }
void clear();
// operators that return iterators
iterator begin() { return data; }
const_iterator begin() const { return data; }
iterator end() { return avail; }
const_iterator end() const { return avail; }
void push_back( const T& val ) {
if ( avail == limit ) grow();
unchecked_append( val );
}
private:
iterator data; // points to beginning of data
iterator avail; // points to end of initialized data
iterator limit; // points to end of data
std::allocator<T> alloc; // object to handle data allocation
void create();
// functions to support push_back()
void grow();
void unchecked_append( const T& );
};
// Creates an empty vector.
template <class T>
void Vec<T>::create() { data = avail = limit = 0; }
// All the elements of the vector are dropped: their destructors are called,
// and then they are removed from the vector container,
// leaving the container with a size of 0.
// The capacity remains the same, however.
template <class T>
void Vec<T>::clear()
{
std::cout << "capacity before clear: " << capacity() << std::endl;
std::cout << "data = " << data << " limit = " << limit << std::endl;
if (data) {
iterator it = avail;
// destroy objects in reverse order
while ( it != data ) {
alloc.destroy(--it);
}
}
data = avail = 0;
std::cout << "capacity after clear: " << capacity() << std::endl;
std::cout << "data = " << data << " limit = " << limit << std::endl;
}
// Controls how the vector should grow if it needs more space.
template <class T>
void Vec<T>::grow()
{
// Allocate twice as much storage as is currently used.
// If matrix is empty, allocate one element.
size_type new_size = std::max( 2*(limit-data), ptrdiff_t(1) );
// Allocate new space and copy existing elements
iterator new_data = alloc.allocate( new_size );
iterator new_avail = std::uninitialized_copy( data, avail, new_data );
// Deallocate old space
uncreate();
// Reset pointers to new values
data = new_data;
avail = new_avail;
limit = data + new_size;
}
// Create space for one element at the end and put given value there.
template <class T>
void Vec<T>::unchecked_append( const T& val )
{
alloc.construct( avail, val );
avail++;
}
我测试这个使用
Vec<int> v;
for ( int i = 0; i < 100; i++ ) {
v.push_back(i);
}
std::cout << "size=" << v.size() << " capacity=" << v.capacity() << std::endl;
v.clear();
std::cout << "size=" << v.size() << " capacity=" << v.capacity() << std::endl;
我得到以下输出：
size=100 capacity=128
capacity before clear: 128
data = 0x100100280 limit = 0x100100480
capacity after clear: 1074004256
data = 0 limit = 0x100100480
size=0 capacity=1074004256
出于某种原因，clear()破坏了limit指针。当它甚至不修改它时，这怎么可能。代码看起来很简单，但我看不到我缺少什么。
谢谢！